The present invention relates to a class of dioxoles, and more specifically it relates to TFE copolymers with dioxoles, said copolymers having a higher Tg in the range 0.1-50% by moles of dioxole, in comparison with the copolymers of TFE with the known dioxoles.
Various structures of dioxoles have been described in the art. The U.S. Pat. No. 3,865,845 describes the perfluorodimethyldioxole a (PDD) having the formula: 
The patent EP 76,581 describes the following class of halo-perfluoroalkyl-dioxoles: 
wherein Y4, Y5 and Y6 are F or Cl, while R2 is a perfluoroalkyl radical having from 1 to 4 carbon atoms.
The patent EP 80,187 relates to 2,2,4,5-tetrafluoro-1,3-dioxole (PD) having the formula: 
The patent EP. 633,257 relates to the following class of perfluoroalkyl-perfluoroalkoxy dioxoles having the formula: 
wherein Rf is a perfluoroalkyl radical having from 1 to 5 carbon atoms; X1 and X2 are, independently from each other, F or CF3.
The drawback of the perfluorodioxoles described in the U.S. Pat. No. 3,865,845, EP 80,187 and EP 76,581 is that they tend to spontaneously homopolimerize.
For example the perfluorodioxole (PD) of EP 80,187 tends to homopolymerize also at very low temperatures such as xe2x88x9278xc2x0 C. Therefore this substance is not stable. A further drawback is that in the copolymerization products it is difficult to have an homogeneous distribution of the components along the chain.
In the GB patent 2,211,845 a process is described to obtain dioxoles having formula: 
wherein R1 and R2 are independently selected from fluorine, chloroalkyl and C1-C3 fluoroalkyl; R3 is fluorine, or chlorine, or C1-C3 perfluoroalkyl. In this patent reference is made to the U.S. Pat. No. 3,865,845 (see above) for the obtainment of dioxolanes from which the dioxoles having the above mentioned formula are obtained. This patent (GB 2,211,845) does not give any indication about the properties of the obtainable copolymers and in particular about the Tg.
In the art, the reactivity of substituted perfluoroalkyl dioxoles that is of perfluoro-2,2,4-trimethyl-1,3,-dioxole (PTD) (Ming-H. Hung, Macromolecules 26, 5829-5834, 1993) having the formula 
has been studied.
This compound is obtained by reacting hexafluoroacetone with 2,3 epoxy-1-propanol (M. H. Hung, J. Can. Chem. Soc. 1990, 112, 9672).
The PTD dioxole shows a poor reactivity: it lowers the polymerization kinetics, it does not homopolymerize and is not capable to copolymerize with TFE. The PTD low reactivity can be attributed to the steric effects of the 4 position trifluoromethyl group.
As regards the processes to obtain (per)fluorodioxoles, according to the art, the preparation of these compounds is carried out from the corresponding dioxolanes having a chlorine atom respectively in the 4 and 5 positions by dehalogenation reaction with metals such as Mg, Zn, in particular Mg. These reactions are carried out in organic solvent such as dioxane, DMF, in particular dimethylformamide (see U.S. Pat. No. 3,865,845, EP 76,581, EP 80,187). Reactions using polluting solvents are involved, whose disposal is difficult. In the art (WO 91/03472) it has moreover been shown that the dehalogenation yield increases when the dioxolane the anti isomer amount (the isomery is referred to the position of the two chlorine atoms in the molecule) is higher than that of the sin isomer.
The need was felt to obtain perfluorodioxoles allowing to prepare copolymers with olefinic comonomers, in particular TFE, having a Tg higher than the copolymers of the art with the same dioxole content.
A higher Tg value allows the use of the polymers at higher temperature.
It has been now surprisingly and unexpectedly found that it is possible to overcome this problem by means of a novel class of perfluorodioxoles, which are obtained with high yields by a new process more favourable to the environment than the known one.
An object of the present invention is a class of perfluoro-4-alkyl-1,3-dioxoles having the general formula: 
wherein Rf is a perfluoroalkylic radical having from 1 to 5 carbon atoms.
Surprisingly said class of dioxoles copolimerizes with olefinic monomers even if the perfluoroalkyl substituent is present in the 4 position of the ring, to the steric effects of which the PTD low reactivity has been attributed, as seen in the art.
In examining the prior art, it has been noticed that factors of steric type, due to the presence of groups directly bound to the carbon atoms in position 4 or 5 of the dioxolene ring, are capable to lessen or hinder the polymerization processes.
Surprisingly, the compounds of formula (I) are, on the contrary, capable to easily (co)polymerize.
The preferred compound in the general formula is perfluoro-4-methyl-1,3-dioxole (PMD) having the formula (II): 
A further object of the present invention are the copolymers with monomers containing olefinic or of olefinic type unsaturations, obtainable with the compounds of formula (I), containing from 0.1 to 50% by moles of the following unit: 
wherein Rf is a perfluoroalkylic radical as above defined.
These polymers show a more homogeneous composition along the chain, also when the dioxoles concentration in the polymer is higher than 12% by moles.
The perfluorodioxoles of the invention can copolymerize with one or more comonomers having oelfinic unsaturations, such as for example, differently from PTD, tetrafluoroethylene.
It is surprising and unexpected that the copolymers with TFE in which the dioxole content of the invention is in the range 0.1-50% by moles have an improved Tg in comparison with the TFE copolymerized with the known dioxoles.
The monomers of the invention can copolymerize with vinyilidene fluoride, vinyl fluoride, trifluoroethylene, perfluoropropene, perfluoromethylvinylether, perfluoroethylvinylether, perfluorodioxole PD, perfluoro (2,2-dimethyl) -1,3-dioxole, perfluoro-4-methoxy-1,3-dioxole, CF2xe2x95x90CFxe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F, CF2xe2x95x90CFxe2x80x94(OCF2 (CF3)CF)nxe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F wherein n is an integer from 1 to 3, chlorotrifluoroethylene, vinyl chloride, methyl (meth)acrylate, butyl (meth)acrylate, ethylene.
The copolymers can be prepared by polymerization of radical type, both in aqueous and organic medium.
In the aqueous medium polymerizations, the polymerization initiator can be any substance capable to produce radicals, such as for instance peroxides, persulphates or azo-compounds. These compounds in the reaction conditions have an average life such as to allow to obtain the polymer with the desired molecular weight. Also a reducing agent can optionally be used, such as for instance an iron salt, in order to promote the initiator decomposition.
The used initiator amount depends, as known, on the polymerization temperature, on the optional transfer agent presence, on the desired molecular weight and generally on the employed reaction conditions. The aqueous medium polymerization requires the presence of an emusifying agent. See for instance EP 184,459.
Alternatively, the polymerizations can be carried out in organic solvent as described in U.S. Pat. No. 3,642,742. Any initiator suitable to the TFE polymerization in organic solvent can be used. Preferably the initiator must be soluble in the reaction solvent. Examples of initiators are alkylpercarbonates, perfluoroacylperoxides, benzoyl peroxide and azo-bis(isobutyronitrile).
Redox systems can also be used such as those described in Prog. Polym. Sci, 8, 61 (1982). The solvent is generally selected from (hydro) (chloro) fluorocarbons and (hydro)perfluoropolyethers, when H is present it is in one or both ends, preferably in both.
The novel polymers have Tg higher than those of the polymers of the art containing the same dioxole percentage, as shown in Table 1.
As already said, higher Tg values allow a greater flexibility of the polymer use at high temperatures.
The TFE crystalline copolymers are used to prepare dielectric materials.
Amorphous copolymers containing the invention dioxoles are used for coating of electric wires and of parts to be insulated, besides in the optical fibers field, due to their low refraction index.
The amorphous copolymers use higher amounts of comonomer in comparison with the crystalline copolymers. This depends on the various types of comonomer used. The skilled in the art can easily determine the invention dioxole amount for obtaining amorphous copolymers by determining with the known analytical methods of the art the disappearance of the crystalline domains. Generally in the crystalline copolymers the % by moles of dioxole is lower than 15% by moles.
A further object of the present invention is a process for preparing with high yields the perfluorodioxoles of the invention, characterized in that the dioxolanes containing in position 4 of the ring a fluorine atom and a second halogen selected from F, Cl and Br, and in position 5 an hydrogen atom, are submitted to dehydrohalogenation reaction in alkaline solution.
When in position 4 a Cl or Br atom is present, it has been found that the dehydrohalogenation reaction is selective towards the removal of HCl or HBr, respectively, also when the dioxolane molecule contains fluorine atoms in vicinal position with respect to the C-H group, and, therefore, HF elimination could take place.
Moreover it has been found that the yields of the new process are independent from the percentages of sin and anti isomers present in the starting dioxolane.
The dehydrohalogenation reaction is carried out in KOH or NaOH aqueous solutions in phase transfer conditions, for example in a KOH solution having a concentration in the range 20-60% by weight, preferably 30-50%; the reaction temperature is in the range 20xc2x0-100xc2x0 C., preferably 30xc2x0-80xc2x0 C. The phase transfer agent can be a phosphonium salt or a quaternary ammonium salt. In this way the problem solution of the reaction mother liquors disposal is made much easier with respect to the prior art.
The perfluoro dioxoles of the present invention can be prepared by reacting, in a first step (reaction a) an olefin of formula Rfxe2x80x94CHxe2x95x90CFX (Xxe2x95x90F, Cl, Br), wherein Rf is a radical as above defined, with hypofluorite gas CF2(OF)2 (BDM) and by subjecting in a second step (reaction b) the obtained dioxolanes to dehydrohalogenation in KOH or NaOH aqueous solutions according to the following process: 
The reaction a) is carried out at temperatures in the range xe2x88x92140xc2x0 C./+60xc2x0 C., preferably xe2x88x92110xc2x0 C./xe2x88x9220xc2x0 C., and leads to the dioxolane obtainment.
The olefin of formula Rfxe2x80x94CHxe2x95x90CFX wherein X=F can be perpared according to the following general scheme (M. Murray et al. J. Am. Chem. Soc. 82, 2868 (1960): 
Instead of the fluorine atom on the carbon in position 1 also a chlorine or bromine atom can be introduced according to the methods known in the art.
The olefin is in liquid phase and preferably is mixed with a solvent selected from (hydro)(chloro)fluorocarbons and/or (hydro)perfluoropolyethers, when H is present it is in one or both terminal ends, preferably in both (ex. Galden(copyright), H-Galdenv(copyright), Fomblin(copyright), Krytox(copyright) Demnum(copyright)). Among the latter, those having a molecular weight comprised between 500 and 1000 are preferred.
The hypofluorite or bis(fluoroxy)difluoromethane (BDM) is preferably fed in a continuous way, in gaseous phase and preferably diluted with an inert gas such as for example N2, He or Ar. The ratio between the volume of the diluent gas and the hypofluorite is generally in the range 3-10, preferably 3-6.
The bis(fluoroxy)difluoromethane is a known compound and can be prepared for example as described in F. A. Hohorst, J. M. Shreeve, J.Am.Chem.Soc.87, 1809 (1967); P. J. Tompson, J.Am.Chem.Soc. 89, 4316 (1967); R. L. Cauble e G. H. Cady, J.Am.Chem.Soc. 87, 5161 (1967).
The dehydrohalogenation reaction b) is carried out as previously described.
The final compounds are distilled from the reaction crude material and, as said, can be indefinitely kept at room temperature.
A further object of the present invention is the synthesis of the dioxole of formula (II) (PMD) according to the following synthesis scheme:
axe2x80x2) radical addition of bromo-chloro-difluoromethane CF2BrCl to 1,1-difluoroethylene (VDF) CH2xe2x95x90CF2 to obtain the compound having the formula BrCF2xe2x80x94CH2xe2x80x94CF2Cl; 
bxe2x80x2) dehydrohalogenation reaction of the compound BrCF2xe2x80x94CH2xe2x80x94CF2Cl in phase transfer conditions in KOH aqueous solution, and formation of the olefin having the formula ClCF2xe2x80x94CHxe2x95x90CF2; 
cxe2x80x2) re-arrangement of the ClCF2xe2x80x94CHxe2x95x90CF2 olefin catalyzed by AlCl3 and formation of the olefin of formula CF3xe2x80x94CHxe2x95x90CFCl; 
dxe2x80x2) addition of bis(fluoroxy)difluoromethane to the CF3xe2x80x94CHxe2x80x94xe2x95x90CFCl olefin with formation of the dioxolane: 
exe2x80x2) dioxole formation by dehydrohalogenation of the dioxolane in KOH or NaOH aqueous solutions: 
The reaction (axe2x80x2) occurs in the presence of initiators such as for example azo-compounds or peroxides; the reaction temperature is selected in connection with the used initiator and generally is in the range 60xc2x0-130xc2x0 C.; the molar ratio between CF2BrCl and VDF is in the range 2-10, preferably 3-6. The presence of solvents is not necessary.
For the use of CF2Br2 as a telogen, reference is made to the paper by P. Tarrant, A. M. Lovelace and M. R. Lilyquist, J. Am.Chem.Soc.77, 2783 (1955).
The reaction (bxe2x80x2) is carried out in phase transfer conditions, in KOH aqueous solution having a concentration in the range 20-60% by weight, preferably 30-50%; the reaction temperature is in the range 20xc2x0-100xc2x0 C., preferably 30xc2x0-70xc2x0 C.; the phase transfer agent can indifferently be a phosphonium salt or a quaternary ammonium salt; the reaction is carried out at the reduced pressure of 600-100 mmHg, preferably 500-200 mmHg. The reduced pressure allows to remove from the reaction medium the ClCF2xe2x80x94CHxe2x95x90CF2 olefin which is obtained with high yields, and which is collected in a trap at low temperature directly connected with the reactor.
The reaction (cxe2x80x2) is carried out with an amount by moles of AlCl3, in the range 2-20%, preferably 5-15% with respect to the olefin moles ClCF2xe2x80x94CHxe2x95x90CF2; the reaction temperature is in the range xe2x88x92500/+30xc2x0 C., preferably xe2x88x9230/+20xc2x0 C. In this step the presence of solvents is not necessary.
The reaction (dxe2x80x2) and the subsequent one (exe2x80x2) are carried out as described for the obtainment of perfluorodioxoles.